Indentation testing to determine the hardness of materials has a long history. Conventional indentation tests include the Brinell hardness test, the Rockwell hardness test, and the Vickers hardness test. The Brinell and Vickers tests involve indenting at a fixed load and then examining the diameter of the indentation. As shown schematically in FIG. 1A, the Rockwell test, which is the most commonly used test, involves measuring the depth of indentation from a fixed load by measuring how far a test probe 102 goes into the material under test 104. This requires a rigid frame 106. It cannot work if there is a soft layer in the mechanical path from the top of the material under test 104 down through the rigid frame 106 and back to the test probe 102 that will deform during indentation (as indicated schematically by the springs 108 in FIG. 1C) because the distance that the test probe 102 goes into the material under test 104 cannot be distinguished from the deflection of the soft layer. A real example of this problem would be attempting to measure the Rockwell hardness of a bone surface exposed during surgery. The soft tissue between the bone and the table on which the body rested would be like the springs 108 shown in FIG. 1C.
The development of very sensitive methods for measuring the depth of indentations such as capacitance sensors, optical beam deflection, laser interferometers or even very sensitive linear variable differential transducers, LVDTs, together with the development of sophisticated techniques for determining mechanical parameters from force vs. distance data only, (ref. W. C. Oliver and G. M. Pharr. Measurement of hardness and elastic modulus by instrumented indentation: Advances in understanding and refinements to methodology. J. Mater. Res. 19 (2004), 3. (review article)), has made possible a new class of indentation machines called nanoindentation testers or nanoindenters. They typically use submicrometer indentations. Nanoindentation testors also use a rigid frame 106 as shown schematically in FIG. 1A to enable accurate measurement of the distance that an indenter goes into the sample at a fixed load for macroindentation tests or variable loads for nanoindentation tests. Again, a substantial soft layer under the sample as shown in FIG. 1C would prevent accurate nanoindentation testing.
This solution to the problem of soft layers has been previously implemented, for example, in U.S. Pat. No. 1,770,045, with a durometer as shown in FIG. 2A. In this case a rigid frame is not needed because the base of the durometer 202 rests directly on the material under test 204 and indentations of the test probe 206 (sometimes called the foot) into the material are measured relative to the position of the base of the durometer 202. However, durometer indentation measurements only characterize the material with a hardness number. Attempts have been made to relate hardness measurements taken with a durometer to the elastic modulus of the material. However, no accurate, widely accepted model is available. This is in part due to the difficulties in theoretical analysis arising from the complex indenter geometry, and the inability to correct for time-dependent effects because of a lack of control of the loading rate with the durometer [Briscoe, B. J. and Sebastian, K. S. An analysis of the durometer indentation. Rubber Chemistry and Technology 66 (5): 827-836 1993)].
Other prior art portable hardness testers also exist. In particular there are many rebound testers such as the TH130 and TH150 pocket-size hardness tester from Corvib and many ultrasonic hardness testers such as the High Resolution SH-21 Portable Hardness Tester from Micro Photonics Inc. Here too, however, to the best of our knowledge there exists no portable tester that measures more material properties beyond just hardness.
One approach to indentation measurement on soft samples is to use, as a distance reference, the upper surface of the sample as is found in the instrument outlined in U.S. Pat. No. 6,142,010. In spite of this improvement, this instrument is limited in that it is solely designed for measuring hardness and relies on an external mechanical frame (as opposed to a reference probe) to maintain a rigid mechanical path between the sample and the distance measurement. The upper surface of the sample is used for a differential measurement of the indentation depth in the CSM Indentation Testers, which can measure more that just hardness. Here again, however, a rigid frame is present.
Atomic Force Microscopes (AFMs) can rest on the surface of the material under test and could, in principle at least, be used for indentation tests [C. A. J. Putman, H. G. Hansma, H. E. Gaub, and P. K. Hansma, Langmuir 8, 3014 (1992)]. An example of indentation tests on bone with the AFM is James B. Thompson et al., Nature 414, 774, 13 Dec. 2001, though this was done with a prototype AFM that was not capable of resting on the surface of the material under test.
One AFM company, Asylum Research, has also produced a nanoindenter, the MFP-3D NanoIndenter™ for Quantitative Surface Characterization. This instrument eliminates the problem of angular motions of cantilevers and goes to higher forces, up to 14 milliNewtons. It consists of a new NPS™ Nanopositioning sensor for their MFP-3D™ Stand Alone Atomic Force Microscope. The sample is held rigidly to the MFP-3D scanner through specialized sample mounts. Thus it is not designed to rest on the surface of the sample as for the present invention.
Other publications dealing with prior art systems include: U.S. Pat. No. 5,450,745; U.S. Patent Publication Nos. 2002/0170360 and 2005/0262685; “Micro Hardness Tester (MHT) for fracture toughness determination of brittle materials”, No. 8, July 1998; CSM Indentation Testers, four page brochure; and “ASTM Proposed Instrumented Indentation Testing Standard”, pages 1-4, October 2003.
Thus, while there have been portable hardness testing devices and devices that measure parameters other than hardness, we are aware of no prior device that combine the ability to be portable with the ability to measure a wide variety of parameters based on indentation of a probe into a sample.